Researchers May Have Found Secret to Durable Cave Art

Researchers may have discovered the secret to long-lasting cave paintings in China: human blood. Scientists examining 2,000-year-old rock paintings in the Zuojiang River Valley say the art's staying power likely comes from a two-layer paint system that mixed quicklime, water, and blood into a mortar-like base, which was then topped with red clay pigment, reports Smithsonian Magazine.

Protein analysis of fallen fragments suggests most of the blood was human, with markers common in pregnant women—raising the possibility that childbirth blood was used, according to the study in the Journal of Archaeological Science. Not everyone is persuaded: Outside experts warn the faint human signals could reflect lab contamination or overinterpretation, and note those proteins aren't exclusive to pregnancy, per Science. Future studies may test whether blood chemistry lines up with fertility-themed images, like pregnant women and sex scenes depicted on the cliffs.